Data transmission apparatus and communication system using a constellation rearrangement

ABSTRACT

A hybrid ARQ retransmission method in a communication system, wherein data packets being encoded with a forward error correction (FEC) technique prior to transmission, are retransmitted based on an automatic repeat request and subsequently soft-combined with previously received erroneous data packets either on a symbol-by-symbol or a bit-by-bit basis The symbols of said erroneous data packets are modulated by employing a predetermined first signal constellation. The symbols of the retransmitted data packets are modulated by employing at least a predetermined second signal constellation. Each symbol bit has a mean bit reliability defined by the individual bit reliabilities over all symbols of the predetermined signal constellation. According to the invention, the predetermined first and the at least second signal constellation are selected such that the combined mean bit reliabilities for the respective bits of all transmissions are averaged out.

This is a continuation of application Ser. No. 11/003,437 filed Dec. 6,2004, which is a continuation of application Ser. No. 101239,794 filedSep. 25, 2002, now U.S. Pat. No. 6,892,341 B2, issued May 10, 2005.

FIELD OF THE INVENTION

The present invention relates to a hybrid ARQ retransmission method in acommunication system.

BACKGROUND OF THE INVENTION

A common technique in communication systems with unreliable andtime-varying channel conditions is to correct errors based on automaticrepeat request (ARQ) schemes together with a forward error correction(FEC) technique called hybrid ARQ (HARQ). If an error is detected by acommonly used cyclic redundancy check (CRC), the receiver of thecommunication system requests the transmitter to resend the erroneouslyreceived data packets.

S. Kaliel, Analysis of a type II hybrid ARQ scheme with code combining,IEEE Transactions on Communications, Vol. 38, No. 8, August 1990 and S.Kallel, R. Link, S. Bakhtiyari, Throughput performance of Memory ARQschemes, IEEE Transactions on Vehicular Technology, Vol. 48, No. 3, May1999 define three different types of ARQ schemes:

-   -   Type I: The erroneous received packets are discarded and a new        copy of the same packet is retransmitted and decoded separately.        There is no combining of earlier and later received versions of        that packet.    -   Type II: The erroneous received packets are not discarded, but        are combined with some incremental redundancy bits provided by        the transmitter for subsequent decoding. Retransmitted packets        sometimes have higher coding rates and are combined at the        receiver with the stored values. That means that only little        redundancy is added in each retransmission.    -   Type III: Is the same as Type II with the constraint each        retransmitted packet is now self-decodable. This implies that        the transmitted packet is decodable without the combination with        previous packets. This is useful if some packets are damaged in        such a way that almost no information is reusable.

Types II and III schemes are obviously more intelligent and show aperformance gain with respect to Type I, because they provide theability to reuse information from of previously received erroneouspackets. There exist basically three schemes of reusing the redundancyof previously transmitted packets:

-   -   Soft-Combining    -   Code-Combining    -   Combination of Soft- and Code-Combining        Soft-Combining

Employing soft-combining the retransmission packets carry identicalsymbols compared with the previously received symbols. In this case themultiple, received packets are combined either by a symbol-by-symbol orby a bit-by-bit basis as for example disclosed in D. Chase, Codecombining: A maximum-likelihood decoding approach for combining anarbitrary number of noisy packets, IEEE Trans. Commun., Vol. COM-33, pp.385-393, May 1985 or B. A. Harvey and S. Wicker, Packet CombiningSystems based on the Viterbi Decoder, IEEE Transactions on.Communications, Vol. 42, No. 2/3/4, April 1994. By combining thissoft-decision values from all received packets the reliabilities of thetransmitted bits will increase linearly with the number and power ofreceived packets. From a decoder point of view the same FEC scheme (withconstant code rate) will be employed over all transmissions. Hence, thedecoder does not need to know how many retransmissions have beenperformed, since it sees only the combined soft-decision values. In thisscheme all transmitted packets will have to carry the same number ofsymbols.

Code-Combining

Code-combining concatenates the received packets in order to generate anew code word (decreasing code rate with increasing number oftransmission). Hence, the decoder has to be aware of the FEC scheme toapply at each retransmission instant. Code-combining offers a higherflexibility with respect to soft-combining, since the length of theretransmitted packets can be altered to adapt to channel conditions.However, this requires more signaling data to be transmitted withrespect to soft-combining.

Combination of Soft- and Code-Combining

In case the retransmitted packets carry some symbols identical topreviously transmitted symbols and some code-symbols different fromthese, the identical code-symbols are combined using soft-combing asdescribed in the section titled “Soft Combining” while the remainingcode-symbols will be combined using code-combining. Here, the signalingrequirements will be similar to code-combining.

As it has been shown in M. P. Schmitt, Hybrid ARQ Scheme employing TCMand Packet Combining, Electronics Letters Vol. 34, No. 18, September1998 that HARQ performance for Trellis Coded Modulation (TCM) can beenhanced by rearranging the symbol constellation for theretransmissions. There, the performance gain results from the maximizingthe Euclidean distances between the mapped symbols over theretransmissions, because the rearrangement has been performed on asymbol basis.

Considering high-order modulation schemes (with modulation symbolscarrying more than two bits) the combining methods employingsoft-combining have a major drawback: The bit reliabilities withinsoft-combined symbols will be in a constant ratio over allretransmissions, i.e. bits which have been less reliable from previousreceived transmissions will still be less reliable after having receivedfurther transmissions and, analogous, bits which have been more reliablefrom previous received transmissions will still be more reliable afterhaving received further transmissions.

The varying bit reliabilities evolve from the constraint oftwo-dimensional signal constellation mapping, where modulation schemescarrying more than 2 bits per symbol cannot have the same meanreliabilities for all bits under the assumption that all symbols aretransmitted equally likely. The term mean reliabilities is consequentlymeant as the reliability of a particular bit over all symbols of asignal constellation.

Employing a signal constellation for a 16 QAM modulation schemeaccording to FIG. 1 showing a Gray encoded signal constellation with agiven bit-mapping order i₁q₁i₂q₂, the bits mapped onto the symbolsdiffer from each other in mean reliability in the first transmission ofthe packet. In more detail, bits i₁ and q₁ have a high mean reliability,as these bits are mapped to half spaces of the signal constellationdiagram with the consequences that their reliability is independent fromthe fact of whether the bit transmits a one or a zero.

In contrast thereto, bits i₂ and q₂ have a low mean reliability, astheir reliability depends on the fact of whether they transmit a one ora zero. For example, for bit i₂, ones are mapped to outer columns,whereas zeros are mapped to inner columns. Similarly, for bit q₂, onesare mapped to outer rows, whereas zeros are mapped to inner rows.

For the second and each further retransmissions the bit reliabilitieswill stay in a constant ratio to each other, which is defined by thesignal constellation employed in the first transmission, i.e. bits i₁and q₁ will always have a higher mean reliability than bits i₂ and q₂after any number of retransmissions.

SUMMARY OF THE INVENTION

The object underlying the present invention is to provide a hybrid ARQretransmission method with an improved error correction performance.This object is solved by a method as set forth in claim 1.

The method subject to the invention is based on the recognition that inorder to enhance the decoder performance, it would be quite beneficialto have equal or near to equal mean bit reliabilities after eachreceived transmission of a packet. Hence, the idea underlying theinvention is to tailor the bit reliabilities over the retransmissions ina way that the mean bit reliabilities get averaged out. This is achievedby choosing a predetermined first and at least second signalconstellation for the transmissions, such that the combined mean bitreliabilities for the respective bits of all transmissions are nearlyequal.

Hence, the signal constellation rearrangement results in a changed bitmapping, wherein the Euclidean distances between the modulation symbolscan be altered from retransmission to retransmission due to the movementof the constellation points. As a result, the mean bit reliabilities canbe manipulated in a desired manner and averaged out to increase theperformance the FEC decoder at the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more in depth understanding of the present invention, preferredembodiments will be described in the following with reference to theaccompanying drawings.

FIG. 1 is an exemplary signal constellation for illustrating a 16 QAMmodulation scheme with Gray encoded bit symbols,

FIG. 2 shows four examples for signal constellations for a 16 QAMmodulation scheme with Gray encoded bit symbols,

FIG. 3 shows an exemplary signal constellation for 64-QAM Gray encodedbit symbols,

FIG. 4 shows six exemplary signal constellations for 64-QAM Gray encodedbit symbols

FIG. 5 is an exemplary embodiment of a communication system in which themethod underlying the invention is employed, and

FIG. 6 explains details of the mapping unit shown in FIG. 5.

DETAILED DESCRIPTION OF EMBODIMENTS

For a better understanding of the embodiments, in the following theconcept of a Log-Likelihood-Ratio (LLR) will be described as a metricfor the bit reliabilities. First the straight forward calculation of thebit LLRs within the mapped symbols for a single transmission will beshown. Then the LLR calculation will be extended to the multipletransmission case.

Single Transmission

The mean LLR of the i-th bit b_(n) ^(i) under the constraint that symbols_(n) has been transmitted for a transmission over a channel withadditive white gaussian noise. (AWGN) and equally likely symbols yields$\begin{matrix}{{{{LLR}_{b_{n}^{i}|r_{n}}\left( r_{n} \right)} = {{\log\left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}{\mathbb{e}}^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack} - {\log\left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}{\mathbb{e}}^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack}}},} & (1)\end{matrix}$where r_(n)=s_(n) denotes the mean received symbol under the constraintthe symbol s_(n) has been transmitted (AWGN case), d_(n,m) ² denotes thesquare of the Euclidean distance between the received symbol r_(n) andthe symbol s_(m), and E_(S)/N₀ denotes the observed signal-to-noiseratio.

It can be seen from Equation (1) that the LLR depends on thesignal-to-noise ratio E_(S)/N₀ and the Euclidean distances d_(n,m)between the signal constellation points.

Multiple Transmissions

Considering multiple transmissions the mean LLR after the k-thtransmission of the i-th bit b_(n) ^(i) under the constraint thatsymbols s_(n) ^((j)) have been transmitted over independent AWGNchannels and equally likely symbols yields $\begin{matrix}{{{{LLR}_{b_{n}^{i}|{\bigcap_{j = 1}^{k}r_{n}^{i}}}\left( {r_{n}^{(1)},r_{n}^{(2)},\ldots\quad,r_{n}^{(k)}} \right)} = {{\log\left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}{\mathbb{e}}^{- {\sum\limits_{j = 1}^{k}{{(\frac{E_{S}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack} - {\log\left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}{\mathbb{e}}^{- {\sum\limits_{j = 1}^{k}{{(\frac{E_{S}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack}}},} & (2)\end{matrix}$where j denotes the j-th transmission ((j−1)-th retransmission).Analogous to the single transmission case the mean LLRs depend on thesignal-to-noise ratios and the Euclidean distances at each transmissiontime.

If no constellation rearrangement is performed the Euclidean distancesd_(n,m) ^((j))=d_(n,m) ⁽¹⁾ are constant for all transmissions and,hence, the bit reliabilities (LLRs) after k transmissions will bedefined by the observed signal-to-noise ratio at each transmission timeand the signal constellation points from the first transmission. Forhigher level modulation schemes (more than 2 bits per symbol) thisresults in varying mean LLRs for the bits, which in turn leads todifferent mean bit reliabilities. The differences in mean reliabilitiesremain over all retransmissions and lead to a degradation in decoderperformance.

16-QAM Strategy

In the following, the case of a 16-QAM system will be exemplarilyconsidered resulting in 2 high reliable and 2 low reliable bits, wherefor the low reliable bits the reliability depends on transmitting a oneor a zero (see FIG. 1). Hence, overall there exist 3 levels ofreliabilities.

Level 1 (High Reliability, 2 bits): Bit mapping for ones (zeros)separated into the positive (negative) real half space for the i-bitsand the imaginary half space the q-bits. Here, there is no differencewhether the ones are mapped to the positive or to the negative halfspace.

Level 2 (Low Reliability, 2 bits): Ones (zeros) are mapped to inner(outer) columns for the i-bits or to inner (outer) rows for the q-bits.Since there is a difference for the LLR depending on the mapping to theinner (outer) columns and rows, Level 2 is further classified:

Level 2a: Mapping of i_(n) to inner columns and q_(n) to inner rowsrespectively.

Level 2b: Inverted mapping of Level 2a: Mapping of i_(n) to outercolumns and q_(n) to outer rows respectively.

To ensure an optimal averaging process over the transmissions for allbits the levels of reliabilities have to be altered by changing thesignal constellations according to the algorithms given in the followingsection.

It has to be considered that the bit-mapping order is open prior initialtransmission, but has to remain through retransmissions, e.g.bit-mapping for initial transmission: i₁q₁i₂q₂

bit-mapping all retransmissions: i₁q₁i₂q₂.

For the actual system implementation there are a number of possiblesignal constellations to achieve the averaging process over theretransmissions. Some examples for possible constellations are shown inFIG. 2. The resulting bit reliabilities according to FIG. 2 are given inTable 1. TABLE 1 Bit reliabilities for 16-QAM according to signalconstellations shown in FIG. 2 Constel- lation bit i₁ bit q₁ bit i₂ bitq₂ 1 High High Low Low Reliability Reliability Reliability Reliability(Level 1) (Level 1) (Level 2b) (Level 2b) 2 Low Low High HighReliability Reliability Reliability Reliability (Level 2a) (Level 2a)(Level 1) (Level 1) 3 Low Low High High Reliability ReliabilityReliability Reliability (Level 2b) (Level 2b) (Level 1) (Level 1) 4 HighHigh Low Low Reliability Reliability Reliability Reliability (Level 1)(Level 1) (Level 2a) (Level 2a)

Moreover, Table 2 provides some examples how to combine theconstellations for the transmissions 1 to 4 (using 4 differentmappings). TABLE 2 Examples for Constellation Rearrangement strategiesfor 16-QAM (using 4 mappings) with signal constellations according toFIG. 2 and bit reliabilities according to Table 1. Scheme 1 Scheme 2Scheme 3 Scheme 4 Trans- (with (with (with (with mission Constel-Constel- Constel- Constel- No. lations) lations) lations) lations) 1 1 11 1 2 2 2 3 3 3 3 4 2 4 4 4 3 4 2

Two algorithms are given which describe schemes using 2 or 4 mappingsoverall. The approach using 2 mappings results in less systemcomplexity, however has some performance degradation with respect to theapproach using 4 mappings. The mapping for i- and q-bits can be doneindependently and, hence, in the following the mapping for the i-bitsonly is described. The algorithms for the q-bits work analog.

16-QAM Algorithms

A. Using 2 Mappings

1. Step (1. Transmission)

Choose Level 1 for i₁

Level 2 for i₂—free choice if 2 a or 2 b

1. Mapping Defined

2. Step (2. Transmission)

Choose Level 1 for i₂

Level 2 for i₁—free choice if 2 a or 2 b

2. Mapping Defined

3. Step

Options:

-   (a) Go to 1. Step and proceed with alternating between 1. and 2.    Mapping-   (b) Use 2. Mapping and proceed with using 2 times 1. Mapping, 2    times 2. Mapping and so on . . .    B. Using 4 Mappings

1. Step (1. Transmission)

Choose Level 1 for i₁

Level 2 for i₂—free choice if 2 a or 2 b

1. Mapping Defined

2. Step (2. Transmission)

Choose Level 1 for i₂

Level 2 for i₁—free choice if 2 a or 2 b

2. Mapping Defined

3. Step (3. Transmission)

Options:

-   -   (a) Choose Level 1 for i₁        Level 2 for i₂ with following options        -   (a1) if in 1. Transmission 2 a was used then use 2 b        -   (a2) if in 1. Transmission 2 b was used then use 2 a    -   (b) Choose Level 1 for i₂        Level 2 for ii with following options        -   (b1) if in 2. Transmission 2 a was used then use 2 b        -   (b2) if in 2. Transmission 2 b was used then use 2 a            3. Mapping Defined

4, Step (4. Transmission)

if option (a) in 3. Step

-   -   Choose Level 1 for i₂        Level 2 for i₁ with following options        -   (a1) if in 2. Transmission 2 a was used then use 2 b        -   (a2) if in 2. Transmission 2 b was used then use 2 a if            option (b) in 3. Step    -   Choose Level 1 for i₁        Level 2 for i₂ with following options        -   (a1) if in 1. Transmission 2 a was used then use 2 b        -   (a2) if in 1. Transmission 2 b was used then use 2 a            4. Mapping Defined

5. Step (5., 9., 13., . . . Transmission)

Choose one out of 4 defined mappings

6. Step (6., 10., 14., . . . Transmission)

Choose one out of 4 defined mappings except

-   -   (a) the mapping used in 5. Step (previous transmission)    -   (b) the mapping giving Level 1 reliability to the same bit as in        previous transmission

7. Step.(7., 11., 15., . . . Transmission)

Choose one out of 2 remaining mappings not used in last 2 transmissions

8. Step (8., 12., 16., . . . Transmission)

Choose mapping not used in last 3 transmissions

9. Step

Go to 5. Step

64-QAM Strategy

In case of a 64-QAM system there will be 2 high reliable, 2 mediumreliable and 2 low reliable bits, where for the low and medium reliablebits the reliability depends on transmitting a one or a zero (see FIG.3). Hence, overall there exist 5 levels of reliabilities.

Level 1 (High Reliability, 2 bits): Bit mapping for ones (zeros)separated into the positive (negative) real half space for the i-bitsand the imaginary half space for the q-bits. Here, there is nodifference whether the ones are mapped to the positive or to thenegative half space.

Level 2 (Medium Reliability, 2 bits): Ones (zeros) are mapped to 4 innerand 2×2 outer columns for the i-bits or to 4 inner and 2×2 outer rowsfor the q-bits. Since there is a difference for the LLR depending on themapping to the inner or outer column/row Level 2 is further classified:

Level 2a: Mapping of i_(n) to 4 inner columns and q_(n) to 4 inner rowsrespectively.

Level 2b: Inverted mapping of 2 a: i_(n) to outer columns and q_(n) toouter rows respectively

Level 3 (Low Reliability, 2 bits): Ones (zeros) are mapped to columns1-4-5-8/2-3-6-7 for the i-bits or to rows 1-4-5-8/2-3-6-7 for theq-bits. Since there is a difference for the LLR depending on the mappingto columns/rows 1-4-5-8 or 2-3-6-7 Level 3 is further classified:

Level 3a: Mapping of i_(n) to columns 2-3-6-7 and q_(n) to rows 2-3-6-7respectively

Level 3b: Inverted mapping of 2 a: i_(n) to columns 1-4-5-8 and q_(n) torows 1-4-5-8 respectively

To ensure an optimal averaging process over the transmissions for allbits the levels of reliabilities have to be altered by changing thesignal constellations according to the algorithms given in the followingsection.

It has to be considered that the bit-mapping order is open prior initialtransmission, but has to remain through retransmissions, e.g.bit-mapping for initial transmission:

i₁q₁i₂q₂i₃q₃

bit-mapping all retransmissions: i₁q₁i₂q₂ i₃q₃.

Analog to 16-QAM for the actual system implementation there are a numberof possible signal constellations to achieve the averaging process overthe retransmissions. Some examples for possible constellations are shownin FIG. 4. The resulting bit reliabilities according to FIG. 4 are givenin Table 3. TABLE 3 Bit reliabilities for 64-QAM according to signalconstellations shown in FIG. 4. Constellation bit i₁ bit q₁ bit i₂ bitq₂ bit i₃ bit q₃ 1 High Reliability High Reliability Middle ReliabilityMiddle Reliability Low Reliability Low Reliability (Level 1) (Level 1)(Level 2b) (Level 2b) (Level 3b) (Level 3b) 2 Low Reliability LowReliability High Reliability High Reliability Middle Reliability MiddleReliability (Level 3b) (Level 3b) (Level 1) (Level 1) (Level 2b) (Level2b) 3 Middle Reliability Middle Reliability Low Reliability LowReliability High Reliability High Reliability (Level 2b) (Level 2b)(Level 3b) (Level 3b) (Level 1) (Level 1) 4 High Reliability HighReliability Middle Reliability Middle Reliability Low Reliability LowReliability (Level 1) (Level 1) (Level 2a) (Level 2a) (Level 3a) (Level3a) 5 Low Reliability Low Reliability High Reliability High ReliabilityMiddle Reliability Middle Reliability (Level 3a) (Level 3a) (Level 1)(Level 1) (Level 2a) (Level 2a) 6 Middle Reliability Middle ReliabilityLow Reliability Low Reliability High Reliability High Reliability (Level2a) (Level 2a) (Level 3a) (Level 3a) (Level 1) (Level 1)

Moreover Table 4 provides some examples how to combine theconstellations for the transmissions 1 to 6 (using 6 differentmappings). TABLE 4 Examples for Constellation Rearrangement strategiesfor 64-QAM (using 6 mappings) with signal constellations according toFIG. 4 and bit reliabilities according to Table 3. Scheme 1 Scheme 2Scheme 3 Scheme 4 Trans- (with (with (with (with mission Constel-Constel- Constel- Constel- No. lations) lations) lations) lations) 1 1 11 1 2 2 3 5 3 3 3 2 6 2 4 4 4 4 6 5 5 5 2 5 6 6 6 3 4

Two algorithms are given which describe schemes using 3 or 6 mappingsoverall. The approach using 3 mappings results in less systemcomplexity, however has some performance degradation with respect to theapproach using 6 mappings. The mapping for i- and q-bits can be doneindependently and, hence, in the following the mapping for the i-bitsonly is described. The algorithms for the q-bits work analog.

64-QAM Algorithms

A. Using 3 Mappings

1. Step (1. Transmission)

1. Step (1. Transmission)

Choose Level 1 for i₁

Choose Level 2 for i₂ (free choice if 2 a or 2 b)

Level 3 for i₃—free choice if 3 a or 3 b

1. Mapping Defined

2. Step (2. Transmission)

Options:

(a) Choose Level 1 for i₂

Choose Level 2 for i₃ (free choice if 2 a or 2 b)

Level 3 for i₁—free choice if 3 a or 3 b

(b) Choose Level 1 for i₃

Choose Level 2 for i₁ (free choice if 2 a or 2 b)

Level 3 for i₂—free choice if 3 a or 3 b

2. Mapping Defined

3. Step (3. Transmission) if (a) in 2. Step

Choose Level 1 for i₃

Choose Level 2 for i₁ (free choice if 2 a or 2 b)

Level 3 for i₂—free choice if 3 a or 3 b if (b) in 2. Step

Choose Level 1 for i₂

Choose Level 2 for i₃ (free choice if 2 a or 2 b)

Level 3 for i₁—free choice if 3 a or 3 b

3. Mapping Defined

4. Step (4., 7., 10, . . . Transmission)

Choose one out of 3 defined mappings

5. Step (5., 8., 11, . . . Transmission)

Choose one out of 3 defined mappings except the mapping used in previoustransmission

6. Step (6., 9., 12, . . . Transmission)

Choose one out of 3 defined mappings except the mapping used in last 2transmissions

7. Step

Go to 4. Step

B. Using 6 Mappings

1. Step (1. Transmission)

Choose Level 1 for i₁

Choose Level 2 for i₂ (free choice if 2 a or 2 b)

Level 3 for i₃—free choice if 3 a or 3 b

1. Mapping Defined

2. Step (2. Transmission)

Options:

(a) Choose Level 1 for i₂

Choose Level 2 for i₃ (free choice if 2 a or 2 b)

Level 3 for i₁—free choice if 3 a or 3 b

(b) Choose Level 1 for i₃

Choose Level 2 for i₁ (free choice if 2 a or 2 b)

Level 3 for i₂—free choice if 3 a or 3 b 2. Mapping Defined

3. Step (3. Transmission)

if (a) in 2. Step

Choose Level 1 for i₃

Choose Level 2 for i₁ (free choice if 2 a or 2 b)

Level 3 for i₂—free choice if 3 a or 3 b

if (b) in 2. Step

Choose Level 1 for i₂

Choose Level 2 for i₃ (free choice if 2 a or 2 b)

Level 3 for i₁—free choice if 3 a or 3 b

3. Mapping Defined

4. Step (4. Transmission)

Choose Level 1 for one bit out of i₁, i₂ or i₃

Choose Level 2 for one out of two remaining bits with followingrestrictions

-   -   (a1) if in one of the previous transmission 2 a was used for        this bit then use 2 b    -   (a2) if in one of the previous transmission 2 b was used for        this bit then use 2 a

Level 3 for remaining bit with following restrictions

-   -   (b1) if in one of the previous transmission 3 a was used for        this bit then use 3 b    -   (b2) if in one of the previous transmission 3 b was used for        this bit then use 3 a        4. Mapping Defined

5. Step (5. Transmission)

Choose Level 1 for one out of two bits not having Level 1 in 4. StepChoose Level 2 for one out of two bits not having Level 2 in 4. Stepwith following restrictions

-   -   (a1) if in one of the previous transmission 2 a was used for        this bit then use 2 b    -   (a2) if in one of the previous transmission 2 b was used for        this bit then use 2 a

Level 3 for remaining bit with following restrictions

-   -   (b1) if in one of the previous transmission 3 a was used for        this bit then use 3 b    -   (b2) if in one of the previous transmission 3 b was used for        this bit then use 3 a        5. Mapping Defined

6. Step (6. Transmission)

Choose Level 1 for bit not having Level 1 in 4. Step and 5. Step

Choose Level 2 for bit not having Level 2 in 4. Step and 5. Step withfollowing restrictions

-   -   (a1) if in one of the previous transmission 2 a was used for        this bit then use 2 b    -   (a2) if in one of the previous transmission 2 b was used for        this bit then use 2 a

Level 3 for remaining bit with following restrictions

-   -   (b1) if in one of the previous transmission 3 a was used for        this bit then use 3 b    -   (b2) if in one of the previous transmission 3 b was used for        this bit then use 3 a

6. Mapping Defined

7. Step (7., 13., 19., . . . Transmission)

Choose one out of 6 defined mappings

8. Step (8., 14., 20., . . . Transmission)

Choose one out of 6 defined mappings except

-   -   (a) the mapping used in 7. Step (previous transmission)    -   (b) the mapping giving Level I reliability to the same bit as in        previous transmission

9. Step (9.,15., 21., . . . Transmission)

Choose one out of 6 defined mappings with giving Level 1 reliability tothe bit not having Level 1 in last 2 transmissions

10. Step (10., 16., 22., . . . Transmission)

Choose one out of 3 remaining mappings not used in last 3 transmissions

11. Step (11., 17., 23., . . . Transmission)

Choose one out of 2 remaining mappings not used in last 4 transmissions

12. Step (12., 18., 24., . . . Transmission)

Choose remaining mapping not used in last 5 transmissions

13. Step

Go to 7. Step

FIG. 5 shows an exemplary embodiment of a communication system to whichthe present invention can be applied. More specifically, thecommunication system comprises a transmitter 10 and a receiver 20 whichcommunicate through a channel 30 which can either be wire-bound orwireless, i.e. an air inteface. From a data source 11, data packets aresupplied to a FEC encoder 12, where redundancy bits are added to correcterrors. The n bits output from the FEC decoder are subsequently suppliedto a mapping unit 13 acting as a modulator to output symbols formedaccording to the applied modulation scheme stored as a constellationpattern in a table 15. Upon transmission over the channel 30, thereceiver 20 checks the received data packets, for example, by means of acyclic redundancy check (CRC) for correctness.

If the received data packets are erroneous, the same are stored in atemporary buffer 22 for subsequent soft combining with the retransmitteddata packets.

A retransmission is launched by an automatic repeat request issued by anerror detector (not shown) with the result that an identical data packetis transmitted from the transmitter 10. In the combining unit 21, thepreviously received erroneous data packets are soft-combined with theretransmitted data packets. The combining unit 21 also acts as ademodulator and the same signal constellation pattern stored in thetable 15 is used to demodulate the symbol which was used during themodulation of that symbol.

As illustrated in FIG. 6, the table 15 stores a plurality of signalconstellation patterns which are selected for the individual(re)-transmissions according to a predetermined scheme. The scheme, i.e.the sequence of signal constellation patterns used formodulating/demodulating are either pre-stored in the transmitter and thereceiver or are signaled by transmitter to the receiver prior to usage.

As mentioned before, the method underlying the invention rearranges thesignal constellation patterns for the individual (re)-transmissionsaccording to a predetermined scheme, such that the mean bitreliabilities are averaged out. Hence, the performance of the FECdecoder 23 is significantly improved, resulting in a low bit error rate(BER) output from the decoder.

1. A transmission apparatus for transmitting data comprising:transmission section that transmits data using one of a plurality ofconstellation versions in a modulation scheme, each of saidconstellation versions defining at least one of (i) bit positions in abit sequence comprising a plurality of bits and (ii) logical values ofthe bits.
 2. A communication system comprising: a transmitting apparatusaccording to claim 1; and a receiving apparatus comprising a receptionsection that receives data transmitted by said transmitting apparatus.